1. Field of the Invention
This invention relates to the measurement of edgewise compressive deformation (creep) of paperboard or other sheet materials, during extended durations of load.
2. Description of the Prior Art
Edgewise compressive deformations is of particular importance in paperboard materials which are used to make corrugated paperboard containers. Evaluation of edgewise compressive properties in sheet materials requires not only means to apply the uniform compressive loads, but also means to provide lateral support to prevent buckling of the test specimen. The method currently used for evaluation of creep require an expensive apparatus and a continuous vacuum source. For this reason, it is not practical to consider evaluating large numbers of specimens at one time. It is also not practical to evaluate material behavior in actual warehouse environements.
Prior art approaches to the problem of measuring compressive behavior can be found in the following references:
Franks, R., and Binder W. O. (1941), "The Stress-Strain Characteristics of Cold-Rolled Austenitic Stainless Steels In Compression As Determined By The Cylinder Test Method", ASTM Proceedings 41; 629.
Setterholm, V. C., and Gertjejansen, R. O. (1965), "Method For Measuring The Edgewise Compressive Properties Of Paper", Tappi 48 (5): 308.
Stockman V. (1976), "Measurement Of Intrinsic Compressive Strength Of Paper", Tappi 59 (7): 93. Tappi Standard T 818 OM82 (1982) Ring Crush of Paperboard.
Fellers, C., and Jonsson, P. (1975), "Compression Strength of Linerboard And Corrugated--An Analysis Of Testing Methods", (Swedish), Svensk Papperstidning 78(5): 172.
Jackson, C. A., Koning, J. W., and Gatz, W. A. (1976), "Edgewise Compressive Test of Paperboard By A New Method", Pulp and Paper Magazine of Canada 77(10): T 180.
Seth, R. S., and Sosynski, R. M. "The Intrinsic Edgewise Compressive Strength Of Paper: An Evaluation Of Methods", Tappi 62(10): 125.
Ramberg, W., and Miller, J. A. (1946), "Determination And Presentation Of Compressive Stress-Strain Data For Thin Sheet Metal", J. of Aeronautical Sciences 13(11): 569.
Calvin, S., and Fellers, C. (1975), "A New Method For Measuring The Edgewise Compression Properties Of Paper", Svensk Papperstidning 78(9): 330.
Gunderson, D. E. (1981), "A Method For Compressive Creep Testing of Paperboard", Tappi 64(11): 67.
North American Rockwell Corporation, (1969), "Structural Design Guide For Advanced Composite Applications", U.S. Air Force Contract F 33615-69-C-1368.
Lauraitis, K. N. (1981), "Fatigue Of Fibrous Composite Materials", ASTM Special Technical Publication 723, ASTM, Philadelphia, Pa.
Tsai, S. W., Halpin, J. C., and Pagano, N. J. "Composite Materials Workshop", Technomic Publishing Co., Stamford, Conn.
Whiteside, J. B., Daniel, I. M., and Rowlands, R. E. (1973), "The Behavior Of Advanced Filamentary Composite Plates With Cutouts", U.S. Air Force Systems Command Technical Report AFFDL-TR-73-48.
Stuart, M. J., and Herakovich, C. T. (1977), "Tensile and Compressive Test Results for Metal Matrix Composites", VPI-E-77-6 Virginia Polytechnic Institute and State University.
Lenoe, E. M. (1970), "Testing And Design Of Advanced Composite Materials," J. of the Eng. Mechanics Div., Proceedings of the Amer. Soc. of Civil Engineers.
The first problem encountered in attempting to measure the compression creep properties of a thin sheet material such as paperboard is that the sheet will buckle at very light loads. A number of methods and devices have been proposed and developed to overcome this problem. These methods involve six general approaches outlined below and described in the above cited references:
1. The sheet is rolled into a tube and provided with internal support.
2. The span (length of specimen) between the supports is shortened to the point where buckling does not occur.
3. The sheet is restrained between two flat plates.
4. The sheet is restrained between an array of fingers on each side of the sample. These fingers deflect with the sample as it undergoes strain in the direction of loading-but provide restraint against lateral (out of plane) motion.
5. The sheet is held in contact with a planar surface by a pressure difference.
6. The sheet is laminated with other materials to form a straight composite beam. Lateral support is provided by gluing the specimen to the composite over its entire length.
The method described here is different in concept from all other methods known to the inventors. A uniform lateral force component (normal to the surface of the sheet) is generated as a result of the edgewise compressive force applied to the sheet and the curvature of the composite beam design. This lateral force component forces the sheet in contact with the elastomer and prevents buckling of the sheet. The tensile forces in the steel backing cause it to form a smooth arc and oppose any tendency to waviness or buckling. The sheet being tested is not glued to the other components in its active region as is necessary in a flat composite beam. The method requires a simple means of applying a pure moment loading. It does not require complex and costly fixtures to provide lateral support. Deformation of the specimen, due to compressive creep, results in a reduction in the radius of curvature of the composite beam-of which the specimen is a part. Because of the rigid design of the end portions of the composite beam, the radius of curvature can be directly related to specimen deformation. Small amounts of creep deformation in the specimen sheet are manifest as large changes in radius of curvature, or diameter, of the composite beam and can, therefore, be measured with a common ruler. Measurements can be made at desired intervals of time without maintaining a continuous reference.